1. Field of the Invention
This invention relates to a method and flocculant for chemical treatment applicable to various kinds of water including waste water.
2. Prior Arts
In water purification and waste-water treatment, various kinds of flocculants are used to remove impurities and other substances dispersed in the water. For example, aluminum sulfate, inorganic low-molecular flocculants such as ferric chloride, inorganic polymer flocculants such as polymerized aluminum chloride (PAC) and organic polymer flocculants such as polyacrylamide are used.
Although aluminum sulfate (Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O) is used most popularly because of relatively low cost, this flocculant is not a highly effective flocculant. Namely, the aggregation velocity is slow, the size of formed flocs is small, and adequate aggregation is not performed especially when the temperature of the treated water is low. Also, aluminum sulfate decreases alkalinity so much that a large amount of basic chemicals or other aggregation aids must be used together.
A solution of ferric chloride (FeCl.sub.3.xH.sub.2 O) offers several aggregation advantages, such as formation of larger flocs and efficient coprecipitation with various kinds of heavy metals. But a solution of ferric chloride is corrosive and does not have high stability. Besides, it must be used with slaked lime, which, in turn, results in the production of a very large quantity of sludge. Baking such sludge tends to cause secondary environmental pollution.
Polymerized aluminum chloride (PAC) (Al.sub.2 (OH).sub.n Cl.sub.6-n) has come to be used extensively in place of aluminum sulfate because of its ability to achieve adequate aggregation even at low temperatures and form flocs relatively rapidly. But, as the production process of PAC involves many intricate conditions difficult to control, the products of uniform quality having the same level of aggregating properties are difficult to obtain. Moreover, the cost of PAC is much higher than aluminum sulfate.
Polyacrylamide (CH(CO)(NH.sub.2)(CH.sub.2).sub.n and other organic polymer flocculants form larger flocs at higher speeds. But they involve a serious shortcoming that the treated water is not always safe.
Furthermore, in such aggregation and precipitation processes stated above, aggregation aids are often used with main flocculants. Activated silicic acid is one of such aggregation aids widely used. But the activated silicic acid has an important drawback in that it cannot maintain its aggregating ability when stored for a long time. Namely, activated silicic acid, which is usually prepared by adding acid to silicic acid, becomes gelled in a short time, and gelled silicic acid can no longer be used as a flocculant. As such, it is only for about one day after preparation that activated silicic acid can be used as an aggregation aid. This is the major drawback in the use of activated silicic acid as an aggregation aid.